A. Technical Field
The present invention relates to: a production process for a particulate water-absorbent resin, in which fine particles or superfine particles form little, and the formation of coarse particles can also be suppressed, and particles having an objective particle diameter range are obtained in a good yield; a novel particulate water-absorbent resin having high properties; and a sanitary material comprising this particulate water-absorbent resin.
B. Background Art
Water-absorbent resins are widely utilized for various uses, such as sanitary articles (e.g. disposable diapers, sanitary napkins, and adult incontinent products) and water-retaining agents for soil, and are produced and consumed in large quantities.
Particularly in recent years, in uses of sanitary articles (e.g. disposable diapers, sanitary napkins, and adult incontinent products), the amount of the water-absorbent resins as used tends to be increased, and the amount of pulp fibers as used tends to be decreased, in order to thin the resultant products. The water-absorbent resins are desired to have a large absorption capacity under a load. On the other hand, there are desired low-cost water-absorbent resins because the amount of the water-absorbent resins as used per sheet of the sanitary articles increases.
The water-absorbent resins are generally provided in a powdery state by carrying out aqueous solution polymerization of water-soluble ethylenically unsaturated monomers to thereby obtain a hydrogel polymer, and then drying and pulverizing this polymer. The above hydrogel polymer is obtained as a bulky structure or an aggregated structure of hydrogel particles, and is usually roughly disintegrated with a pulverizer such as a kneader and a meat chopper. The hydrogel as roughly disintegrated is dried until its solid component concentration reaches about 95 weight %, and thereafter pulverized with a pulverizer and then sieved with a classification machine, whereby the particle diameters of the hydrogel are adjusted into a predetermined range. As a result, particulate water-absorbent resins are obtained. Although there is a difference depending upon the uses, there are usually favorably used water-absorbent resins having particle diameters of 150 to 850 μm (but excluding 850 μm) as the water-absorbent resins for sanitary articles.
Hereupon, in addition to the particles having the predetermined particle diameter range, particles with particle diameters of smaller than the predetermined particle diameter range are concomitantly generated in the pulverization step after the drying step. When the water-absorbent resins contain what is called fine powder such as fine particles with particle diameters of smaller than 150 μm or superfine particles with particle diameters of smaller than 106 μm (so-called a fine powder), there are disadvantages in sanitation because of causing such as powder scatter when the water-absorbent resins are handled, and further there are disadvantages in that the lowering of properties (such as lowering of absorption capacity of the water-absorbent resins under a load or lowering of liquid permeability) also occurs. However, the removal of this fine powder results in lowering a yield in the production and leads to the rise of cost. Therefore, techniques of recycling the fine powder are usually introduced. However, in the case of a large amount of the fine powder after the pulverization step, it takes much time to sieve the fine powder, or the enlargement of sieving-apparatuses is caused. Even if the recycling is carried out, operational difficulties increase, and the lowering of productivity and the rise of cost are caused. Therefore, it is necessary to reduce the amount of the fine powder as contained in a period of from after the pulverization step till before the sieving classification procedure. On the other hand, the sieving of particles which are contained after the pulverization step and have particle diameters of not smaller than the predetermined range (so-called coarse particles) is very easy in comparison with that of the fine powder, but the resultant sieved particles are usually pulverized again after the sieving step. Therefore, it is preferred that the amount of the coarse particles should also be small. That is to say, as to the particles as obtained in a period of from after the pulverization step till before the sieving classification procedure, it is desired that: the yield of particles having the predetermined particle diameter range (in other words, having a narrow particle diameter range) is high, and further the fine powder and the coarse particles are sufficiently reduced. Accordingly, the development of such a pulverization step has been demanded.